Pathologists use the word differentiated to describe the difference between the cancer cells and the surrounding normal, health cells (non-cancerous) cells. Poorly differentiated cells look very abnormal compared to normal cells. Cancer cells can be described as poorly differentiated based on their shape, size, or colour.
The opposite of poorly differentiated is well differentiated. Well differentiated cancer cells look very similar to normal, healthy cells although they still behave like a cancer.
Because poorly differentiated cells look so abnormal, pathologists often have to perform additional tests such as immunohistochemistry before they can make a final diagnosis. This is especially important if the poorly differentiated cancer cells are found in a lymph node or other metastatic location.
Why is this important? Poorly differentiated cancers tend to behave in a much more aggressive manner and are associated with worse prognosis than well differentiated cancers.